With a device using a convergent charged particle beam (probe beam), such as the scanning electron microscope (SEM), an ion beam machining device (FIB), and so forth, an observation of image or machining of a specimen is executed by causing a probe to scan over the specimen. In the case of those charged particle radiation devices, resolution or machining accuracy is dependent on a size of the probe in section (a probe diameter), so that the smaller the probe diameter is, the higher the resolution, or the machining accuracy, can be, in theory, enhanced.
Progress has lately been made in development of an aberration corrector for use in a convergent charged particle beam applied device, leading to progress in commercialization of the aberration corrector. With the aberration corrector, rotationally asymmetrical electric field and magnetic field are applied to a beam by use of a multipole lens, thereby giving reverse aberration to a probe beam. By so doing, it is possible to cancel out various types of aberrations including spherical aberration, and chromatic aberration, occurring to an objective lens, a deflection lens, and so forth, in an optical system.
With the optical system of a conventional convergent charged particle beam applied device, axially rotational symmetry lenses are used, and if the optical axes of the respective lenses are aligned with axes of the respective lenses, and astigma as well as focus of the objective lens is adjusted, it is, in theory, possible to adjust the probe diameter so as to be at the minimum value. Further, at the time of executing focus adjustment and astigma correction, images of the probe have been obtained under varied focus conditions, and while comparing the respective images in at least two directions with each other in respect of sharpness, adjustment has been made by selecting the sharpness at the highest point.
On the other hand, in the case of a convergent charged particle beam applied device provided with an aberration corrector, the rotationally asymmetrical electric field and magnetic field are applied by the aberration corrector using a multipole lens. By so doing, with those convergent charged particle beam applied devices, influence of the higher-order aberration becomes prominent while the conventional rotationally symmetrical optical system has no such influence. In order to make the most of the performance of the convergent charged particle beam applied device, it is necessary to find out types of aberrations contained in the beam (aberration components), including those aberrations described as above, and to make accurate measurement on respective amounts of the aberration components, thereby removing all the aberration components by adequately adjusting the aberration corrector.
As one of methods for such a purpose, there is available a method whereby an electron beam falling on a specimen is tilted to get an image, and defocus as well as astigma of the image is measured to thereby find aberrations contained in the beam at the time when the beam is not tilted. In, for example, Non Patent document 1, and so forth, there has been disclosed the principle behind this method with reference to a transmission electron microscope (TEM). More specifically, in the case of the TEM, a method is adopted whereby a ring pattern appearing by Fourier transformation of an image of a specimen of an amorphous structure is analyzed to thereby find defocus and astigma. However, with a convergent charged particle radiation device, it is not possible to get an image having information on such a ring pattern as obtained in the TEM, the method as it is cannot be applied thereto. In order to apply this principle to the convergent charged particle radiation device, there is the need for another technique for measuring defocus, and astigma.
Meanwhile, as one of potent means for measuring defocus, and astigma in the case of the convergent charged particle radiation device, there is available a method whereby an image is obtained at plural focus positions, sharpness of the respective images on a direction-by-direction basis is evaluated, and defocus and astigma are evaluated on the basis of a peak value of the sharpness. This technique has already been known from Patent document 1, and so forth, as a method for automatic focus adjustment, and a method for automatic astigma correction in the case of the charged particle radiation device.
Further, in Patent document 2, there has been disclosed a method whereby evaluation means with varying detection accuracy of focus position are provided, and automatic focusing is carried out by combination of two different evaluation values. Still further, in Patent document 3, there has been disclosed an automatic astigma adjustment method whereby image sharpness is calculated, and an adequate astigma correction direction is selected from an image sharpness angular component value, thereby performing astigma correction.